1. Field of the Invention
This invention is directed to a Hall sensor configured linear displacement transducer which contains magnet that is float activated by way of a spring-float system with precise magnitudes of liquid buoyancy, spring constants, and float density. Together, the linear displacement transducer and the spring-float system combine to form a submersible head unit of this device. The analog output voltage of each of the Hall sensors within the head unit are communicatively connected (either by wired or by wireless connection) to a remote electronic signal conditioner. Depending upon the operational configuration of the floats in the system and the original initialization of the device, the electronic conditioned signal provides two possible measures. When the head unit is configured as a liquid density meter and is completely immersed in a liquid, the combination of the submersible head unit and the electronic signal conditioner constitutes the liquid density meter that provides for measuring and monitoring the density (specific gravity) of the liquid. When the head unit is configured as liquid level meter and is immersed in a container which hold the liquid, the combination of the submersible head unit and the electronic signal conditioner constitutes the liquid level meter that provides for measuring and monitoring the level (height) of the liquid in the container.
2. Description of the Related Art
There are many different types of devices that use different technologies to measure either the density of a liquid or the level of a liquid in a container. In this invention, the basic operating principle of either application can be categorized as being a float-activated device that is responsive to the buoyancy force of the float in the liquid. From physics, the buoyancy force produced by a float in a homoglneous liquid is equal to the weight of the liquid that is displaced by the float. Since buoyancy force is linear with respect to the density of the liquid within which the float is submerged, the measure of the buoyancy force yields a measure of the density of the liquid. Further, since buoyancy force is linear with respect to the volume of liquid displaced by the float within the liquid, then the measure of buoyancy force of a float within a liquid within a container yields a measure of the level (height) of the liquid within the container.
The measure of buoyancy force produced by a float in a liquid is generally dependent upon the measure of the displacement of the float within the liquid by way of a displacement transducer. The liquid density/liquid level meter in this invention uses a distinctive linear displacement transducer that is based on the operating principles of Hall sensors. This distinctive linear displacement transducer is coupled with a spring float system whose parameters meet precise performance criteria.
Discussed below are many conventional liquid density or liquid level measuring devices that are based on the principle of a float-activated system. In a general sense, they differ from each other in regards to the different transducer means that each one utilizes for detecting the relative position of the float in the liquid.
In U.S. Pat. No. 3,089,502, a conventional hydrometer bulb acts as a float that activates the position of a magnetic core of a differential transformer whose output voltage monitors the position of the bulb so as to measure the liquid density.
In U.S. Pat. No. 3,954,010, a float activates the on/off position of the sight line of a beam of light into a light sensor and this is the means for detecting the position of the float in the liquid.
Both U.S. Pat. No. 3,964,317 and U.S. Pat. No. 4,400,978 utilize a similar principle whereby a float activates the position of an electrical sensing coil that is in the vicinity of a stationary magnet. A force-balance restoring current is established in the coil to restore the coil to a neutral position and the magnitude of this current is a measure of the buoyancy force and density of the liquid.
In U.S. Pat. No. 4,015,477, a float activates the position and radius of curvature of a magnetostrictive sensitive wire that is electrically sensed to measure the displacement of the float and, thus, the buoyancy force of the liquid.
In U.S. Pat. No. 4,981,042, a float activates the position of a lever that is connected to the float by way of a pivot assembly. The position of the lever is sensed electronically to measure the displacement of the float to determine the buoyancy force and, thus, the density of the liquid.
In U.S. Pat. Nos. 5,253,522 and 5,471,873, a float activates the position of a toroidal magnet that surrounds a sonic waveguide. A reference magnet also surrounds the waveguide and is positioned at a distance away from the float magnet. An electrical impulse wave is sent along the waveguide and both magnets provide a reflected torsional (magnetostrictive) pulse. The time difference between the reflected signals from the two magnets is indicative of the relative position of the float in the liquid and, thus, the buoyancy force and density of the liquid.
In U.S. Pat. No. 5,447,063, a float activates the tilt of one end of a balance beam. A sensor employs a differential transformer to detect the magnitude of the tilt, thus providing a measure of the buoyancy force and density of the liquid.
In U.S. Pat. Nos. 5,744,716 and 5,847,276, a float activates a pair of force transducers that give a direct measure of the buoyancy force and, thus, the density of the liquid.
U.S. Pat. No. 4,920,797 describes a liquid level sensor with a spring-float assembly and a displacement transducer. In U.S. Pat. No. 4,920,797, two springs hold a single float, which activates the position of a magnet in the vicinity of a Hall sensor.
In addition to the principle of float-activated systems, there are many other different technical approaches to measuring either liquid density or liquid level. Briefly, they are refractive index, vibrating tube, capacitive, vibrating plate, vibrating pipe, radiation, differential pressure, Coriolis meter, inductance coil, and bubble probe.
This invention utilizes a linear displacement transducer whose operating principle is based on the voltage-generating characteristic of Hall sensors. Hall sensors are devices that produce a voltage that is proportional to the magnitude of the transverse magnetic field that intercepts the sensitive plane of the sensor. Edwin H. Hall first discovered the Hall principle (reference: R. P.Winch, Electricity and Magnetism, 1963, Prentice Hall) in 1879.
The Hall sensor linear displacement transducer acts as the proximity-sensing element in the operation of this invention. Hall sensors (two or more) are configured on a non-magnetic sensor fixture assembly such that the sensors are equal-angularly spaced about the circular periphery of the cylindrical sensor fixture. Within the sensor fixture is a concentric borehole that allows for a nonmagnetic actuator rod to move axially between the Hall sensors. An axially aligned permanent magnet is embedded within the actuator rod. The Hall sensors provide an output voltage that is proportional to the magnitude of the radial (transverse) magnetic field that intercepts the sensitive plane of the sensor. As the magnet moves axially in a lateral slide-by approach with respect to the sensors, each of the sensors generates an output voltage that is continuous and linear with respect to the displacement.
A cylindrical (or a rectangular) permanent bar magnet is used in this invention to provide the magnetic field for the Hall sensors. The magnetic field associated with a permanent magnet depends upon the geometry and the magnetization of the magnet. In addition, the magnitude and the direction of the magnetic field depend upon the point of observation with respect to the magnet. From a head-on observation of the field on the longitudinal axis at a distance from one of the poles of the magnet, the field has a direction that is parallel to the longitudinal axis and a magnitude that drops off non-linearly as the reciprocal of the square of the distance. On the other hand, at the lateral sides of the magnet, within the confines of the ends (within 80% of the magnet length), the field has a radial (transverse) component that is linear with respect to the longitudinal axial distance as measured from the center of the magnet. It is this field characteristic that is utilized in the preferred embodiment as the actuator rod moves the magnet in a lateral slide-by approach between the Hall sensors within the sensor fixture assembly.
As the magnet moves axially within the confines of the sensor fixture, the lateral alignment between the magnet and each Hall sensor varies because of possible misalignments of the machine-produced mechanical parts in the assemblyxe2x80x94it is extremely difficult if not impossible to produce machined parts that have perfect alignment and dimensions. For the special case of two sensors positioned diametrically opposite each other, as the actuator rod moves the magnet through the sensor fixture, there are instances when the magnet pushes towards one Hall sensor while at the same time it pulls away from the other. This effect gives rise to the two diametrically opposite Hall sensors producing a push-pull variation in output voltagesxe2x80x94the push-pull effect. To compensate for any of this built in misalignment, the output voltages of the Hall sensors used in this invention are mixed in a dedicated signal conditioner that averages the voltages, consequently canceling out this push-pull effect. Two or more Hall sensors can be configured within and about the sensor fixture to compensate for this push-pull variation by adhering to the constraint that the sensors be equal-angularly spaced about the circular periphery of the fixture; i.e. for two sensors, 180 degree spacing, for three sensors, 120 degree spacing, etc.
When the submersible head unit is immersed in the liquid, the spring-float system provides the mechanism that moves the magnet into the proximity detection region of the linear displacement transducer described above. There are two possible configurations of the spring-float system which gives rise to two possible operating applications of this device.
In the liquid density application-configuration, where the device is used to measure the density of a liquid, two cylindrical floats are coupled together with the actuator rod in-between the two. In the liquid level application-configuration, where the device is used to measure the level of a liquid, only a single cylindrical float is used to hold the actuator rod on one end. In both configurations, springs are used to constrain the movement of the float-rod system within the cylindrical tube that contains the liquid. Guides force the actuator rod, to ride concentrically within the interior of the borehole of the sensor fixture. When the spring-float system is immersed vertically in a liquid, it forces the rod to rise in accordance to the combination values of the buoyancy force of the liquid, the spring constants, and the density of the floats (float). The rise of the floats (float) and the attached actuator rod produces a displacement of the embedded magnet into the proximity detection region of the linear displacement transducer.
In the liquid density application-configuration, where the device is used to measure the density of a liquid, since the displacement of the spring supported floats is linear with respect to the buoyancy force, and the buoyancy force of the liquid is linear with respect to the density of the liquid, then the displacement of the magnet is linear with respect to the density of the liquid. The transducer converts this displacement, by way of a dedicated signal conditioner, into an equivalent linear voltage that is a direct measure of the density (specific gravity) of the liquid.
The liquid density measuring configuration of this invention is based upon the floatation principle of a spring-float system, with an actuator rod and an embedded magnet, that has a preferred mathematical combination of values of buoyancy force, float density, and spring constant. The spring-float system parameters are mathematically evaluated so as to meet the following precise performance criteria: the combined buoyancy force produced by the floats and actuator rod, when totally immersed in pure water (specific gravity equal to 1), equals the combined sum of the gravitational weights of the floats and the actuator rod and the magnet, when measured in air, plus the total spring force exerted on the floats when this combination hangs on the two springs in air.
In the liquid level application-configuration, where the device is used to measure the level of a liquid, since the displacement of the spring supported float is linear with respect to the buoyancy force of the liquid, and the buoyancy force of the liquid is linear with respect to the level of penetration of the cylindrical float within the liquid, then the displacement of the magnet is linear with respect to the level of the liquid in the container. The transducer converts this displacement, by way of a dedicated signal conditioner, into an equivalent linear voltage that is a direct measure of the continuous level (height) of the liquid within the container.
The liquid level measuring configuration of this invention is based upon the floatation principle of a spring-float system, with an actuator rod and an embedded magnet, that has a preferred mathematical combination of values of buoyancy force, float density, and spring constant. The spring-float system parameters are mathematically evaluated so as to meet the following precise performance criteria: the combined buoyancy force produced by the float and actuator rod, when totally immersed in the particular liquid, equals the combined sum of the gravitational weights of the float and the actuator rod and the magnet, when measured in air, plus the total spring force exerted on the float when this combination hangs on the two springs in air.
The coupling together of the spring-float system and the linear displacement transducer described above constitute the submersible head unit of this invention. The mechanical and geometrical configuration of the spring-float system and the linear displacement transducer is provided in the detailed description of the preferred embodiments below. The analog output voltages from the Hall sensors within the linear displacement transducer are input, by way of electrical wires (or by wireless transmission in an alternative configuration), into a remote electronic signal conditioner. Depending upon the configuration of the head unit as a liquid density meter or a liquid level meter, the conditioned electronic signal gives a particular application measurement. As a liquid density meter, the conditioned signal provides a direct measure of the density (specific gravity) of the liquid. As a liquid level meter, the conditioned signal provides a direct measure of the continuous level (height) of a liquid in a container. A thermocouple (temperature sensor) can be attached to the submersible head unit of this invention, thus allowing for monitoring the temperature of the liquid.